The structural evolution and electronic mechanism for KBH4 phase transition have been systematically investigated using first-principles calculations. The present results demonstrate experimentallyobserved stability order of alpha-KBH4 > beta-KBH4 > gamma-KBH4, and the predicted transition pressure at 0 K for cubic -> tetragonal -> orthorhombic phase transformation agrees reasonably with experimental measure. The atomic process indicates that KBH4structural evolution is closely related to rotation and deformation of [BH4]-clusters as well as shift of [BH4]-clusters and K atoms. Along with phase transformation, the band gap of KBH4 narrows, the B-H covalent bond weakens while K-H interaction enhances, the K-K anti-bonding and coulomb repulsion are also stronger. (C) 2014 Elsevier B.V. All rights reserved.